1. Field of the Industrial Application
The present invention relates to a method for continuous casting of a steel slab, and more particularly to a method for continuous casting of a slab wherein a wave of a molten steel surface is depressed by introducing an electro magnetic force to the molten steel in a mold.
2. Description of the Related Art
Molten steel is usually poured from a tundish into a mold through an immersion nozzle to prevent the molten steel from being oxidized. The immersion nozzle prevents the molten steel from being exposed to the air. The immersion nozzle for continuous casting of a slab has a pair of exit ports having openings at its lower end. Molten steel is poured into a mold through the exit ports of the immersion nozzle positioned at the center of the mold toward the circumference inside the mold.
It has been an object in recent years in the field of continuous casting of steel to increase casting speed, namely, the speed of pouring molten steel into a mold for increasing a productivity of a continuous casting machine. However, when the casting speed is increased to more than 1.5 m/min, molten steel in the mold is violently disturbed. Various waves of the molten steel of wavelengths from several meters down to several centimeters are generated on the surface of the molten steel, with a portion of the immersion nozzle being fulcrum, whereby the wave height of the molten steel becomes large. Mold powder is also entangled in the molten steel by such a wave of the molten steel surface. The mold powder entangled in the molten steel and non-metallic inclusions produced during a refining process are prevented from rising up to the surface or the molten steel by the violent disturbance of the molten steel in the mold. As a result, those inclusions are hard to remove from the molten steel in the mold. The inclusions entangled in a slab will appear as surface defects and inner defects of a product that has passed through a final processing. Those surface defects and inner defects of a product greatly the lower quality of the product.
As a prior art to prevent such inclusions from being entangled in a slab, a method for electromagnetically stirring molten steel in a mold, which is disclosed in Japanese Examined Patent Publication No. 10305/89, can be pointed out. In the prior art, an electromagnetic stirrer is placed near the meniscus on a wide side of a mold in a continuous casting apparatus. An electromagnetic inducing force is applied to the molten steel in such a direction so as to force back the molten steel along a direction of a width of the mold from a narrow side of the mold toward the immersion nozzle by means of the electromagnetic stirrer. A flow speed of the molten steel poured into the mold from the immersion nozzle is decreased. Owing to the decrease of the flow speed, the wave motion of the molten steel surface in the mold is decreased and a disturbance of the molten steel therein is depressed.
The magnetic field generator used in the prior art is of a linearly shifting magnetic field type. Therefore, an appropriate value and a frequency of electric current should be determined. The frequency has been determined as follows:
The Lorentz force acting on a poured stream of the molten steel should be enhanced to elevate the damping ratio of the flow speed of the poured molten steel. To enhance the Lorentz force, a relative speed of the poured stream of molten steel to a magnetic flux from the narrow side of the mold toward the immersion nozzle should be increased. Accordingly, a shifting speed of the magnetic flux, that is, a frequency of the magnetic flux should be increased. However, when the frequency of the magnetic flux is increased, the magnetic permeability of stainless steel and mold copper plate composing a frame of the mold is lowered, and the magnetic permability of the molten steel is also lowered. As a result, the density of the magnetic flux acting effectively on the poured stream of the molten steel from the immersion nozzle is decreased. A frequency of 0.5 Hz has customarily been used as the appropriate frequency satisfying a condition of both Lorentz force and the magnetic permeability.
FIG. 1 is a graphical representation showing the magnitude of a wave of a molten steel surface in a mold, when the value of electric current in a magnetic field generator is varied under the condition of electric current frequency of 0.5 Hz in the magnetic field generator. The direction of shift of a magnetic field is the direction from the narrow side of the mold toward the immersion nozzle. The magnitude of the wave is represented with an average value of the amplitude of wave of a the molten steel surface, which are obtained by measuring the amplitude of the wave of the molten steel for ten minutes, at positions 40 mm away from the narrow side of the mold and 40 mm away from the wide side of the mold. As shown in FIG. 2, the wave motions are substantially composed of a short period wave 30 having a period of about 1 to 2 seconds and a long period wave 31 having a period of about 10 to 15 seconds. The amplitude of the wave of the molten steel is a wave height difference 32 between two wave heights. One is a wave height showing the maximum height of the short period wave at a moment closest to a moment when the long period wave shows the maximum height, and the other is a height of wave showing the minimum height of the short period wave at a moment when the long period wave shows the minimum height. Lines A, B, C and D in FIG. 1 were carried out under the following conditions.
In line A, a mold had a width of 850 mm. An immersion nozzle had square openings each directed downwardly at 35.degree. relative to a horizontal line. A casting speed of molten steel was 1.6 m/min. In line B, a mold had a width of 1050 mm. An immersion nozzle had square openings each directed downwardly at 35.degree. relative to a horizontal line. A casting speed of molten steel was 1.8 m/min. In line C, a mold had a width of 1250 mm. An immersion nozzle had square openings each directed downwardly at 45.degree. relative to a horizontal line. A casting speed of molten steel was 2.3 m/min. In line D, a mold had a width of 1350 mm. An immersion nozzle had square openings each directed downwardly at 45.degree. relative to a horizontal line.
A casting speed of molten steel was 2.0 m/min. In any of the cases of the lines A, B, C and D, a frequency in a magnetic field generator was 0.5 Hz.
Under the conditions of A and B that the casting speed of molten steel is comparatively small and the width of the mold is small, as electric current in the magnetic field generator is increased, the effect of depressing the wave of the molten steel surface increases. But, under the conditions of C and D that the casting speed of molten steel is comparatively large and the width of the mold is large, when electric current in the magnetic field generator is excessively increased, the effect of depressing the wave of the molten steel decreases, which promotes the increase of the wave motions, contrary the goals of the technique.